Vertical shaft internal combustion engines are becoming increasingly popular for use in lawn tractors. Their vertical shaft drives grass cutting blades without the use of a costly transmission. Consumer preferences, however, currently dictate lawn tractors with a low hood line. In a vertical shaft engine, this requires a short compact configuration. Even in larger tractors, such as those requiring an engine having 16 hp -35 hp, a low hood line is important to consumers. These larger engines, generate a significant amount of heat during operation and are typically liquid cooled. Liquid cooled vertical shaft engine are not easily shortened because of the necessity of a radiator to cool the liquid cooling the engine.
A liquid cooled engine radiator should be exposed to an air flow in order to operate properly. This radiator must include sufficient surface area in order to adequately cool the engine. In a typical vertical shaft engine, such as shown in U.S. Pat. No. 4,756,280, the radiator has a generally flat, rectangular shape and is disposed above an axial fan mounted on the engine vertical shaft. The shaft rotation causes the axial fan to draw air through the radiator to enhance the rate of heat transfer. This configuration requires a space between the radiator and engine for the fan which increases the overall height of the engine. In addition, sufficient space must be provided to allow the fan to generate an air flow, further increasing the engine height.
Furthermore, a flat radiator with an axial flow fan has a high heat transfer efficiency within the radiator area defined by the fan diameter. However, the area of the radiator outside of the fan diameter, such as the radiator corners, has a significantly lower heat transfer efficiency. In order to provide sufficient cooling, the radiator, must therefore be sized large enough to take into account the varying heat transfer efficiencies in the radiator.
In order to overcome varying heat transfer efficiencies across a flat radiator, round radiators in cooperation with an axial fan have been disclosed, such as in U.S. Pat. No. 4,136,735. In this patent, a round radiator encircles a plenum. An axial fan disposed at the plenum edge either pressurizes the plenum forcing air through the radiator, or creates a suction in the plenum drawing air through the plenum. When the axial fan is adapted to pressurize the plenum, guides disposed about the fan periphery aid the fan in diverting the axial flow of air through the fan to a radial air flow toward the radiator. The round radiator may have an improved heat transfer efficiency in comparison to a flat radiator. However, the plenum in cooperation with the axial fan does not provide a compact engine because of the space requirements for the plenum and fan which increases the overall length or height of the engine.
Therefore, it is desirable to provide a cooling apparatus for a liquid cooled vertical shaft internal combustion engine which provides efficient heat transfer and a compact engine.